The neurotransmitter acetylcholine mediates a variety of responses within the central nervous system and plays an important role in memory function and cognition. Cholinergic responses are mediated by muscarinic and nicotinic receptors throughout the brain, although it is accepted generally that receptors in the cerebral cortex and hippocampus are associated with memory and cognitive function. Agents that block acetylcholine activity at muscarinic receptors and lesions of cholinergic projections to the cortex and hippocampus impair memory and cognition.
In humans, the nucleus basalis of Meynert is the source of acetylcholine for the cerebral cortex and hippocampus. The cholinergic cells within the basal nucleus degenerate in Alzheimer's disease, a disorder that is associated with memory dysfunction and progressive cognitive decline. Current therapeutic approaches for Alzheimer's disease include treatment with agents that increase levels of acetylcholine or mimic the effects of acetylcholine at receptors.
Efforts to increase acetylcholine levels have focused on increasing levels of choline, the precursor for acetylcholine synthesis, and on blocking acetylcholinesterase (AChEase), the enzyme that metabolizes acetylcholine. The first approach, using either choline or phosphatidylcholine, has not been very successful although acetylcholinesterase inhibitors have shown some therapeutic efficacy. Clinical trials with these compounds have documented some improvements in cognitive function and ability to conduct daily tasks. Major drawbacks with AChEase inhibitors include toxicity and the side effects associated with activation of receptors in the peripheral nervous system.
Recent efforts have focused on treating Alzheimer's patients with agonists for muscarinic cholinergic receptors. Natural products, such as the arecoline and pilocarpine ligands, can mimic the effects of acetylcholine at receptors in the central nervous system and reverse cognitive impairments in experimental animals. The clinical application of such ligands is hampered however by the low intrinsic activity of these compounds and their rapid metabolism. Other muscarinic agonists with higher efficacy are not suitable due to either low bioavailability or profound side effects associated with peripheral activity.
Recent molecular biological studies have cloned five subtypes of muscarinic receptors, each with a unique amino acid sequence, tissue-specific expression, ligand binding profile and associated biochemical response. Each subtype is expressed within the central nervous system, although m1, m3 and m4 receptors predominate in the cerebral cortex and hippocampus. In peripheral tissues, the heart expresses m2 receptors while m3 receptors are found in exocrine glands. Pirenzepine, AF-DX 116 and p-F-hexahydrosiladifenidol are selective antagonists for M.sub.1, M.sub.2 and M.sub.3 receptors respectively. Three subtypes (m1, m3 and m5) couple selectively to the stimulation of phosphoinositide metabolism while m2 and m4 more efficiently inhibit adenylyl cyclase.
In addition to the recent studies showing the preferential localization of M.sub.1 receptors in the cerebral cortex and hippocampus, recent findings also show that M.sub.1 antagonists, such as pirenzepine, produce memory impairments in experimental animals.
Thus, it will be appreciated by those skilled in the art that what is needed in the art to reverse the cognitive and memory deficits associated with a loss of cholinergic neurons, as found in Alzheimer's disease, is a selective muscarinic agonist with high central nervous system activity. This agonist should bind selectively to M.sub.1 muscarinic receptors, localized predominantly in the cerebral cortex and hippocampus. It should stimulate phosphoinositide metabolism in the hippocampus.
Even more broadly, however, there is a need in the art to provide muscarinic agonists which have activity at various muscarinic receptor subtypes in the central and peripheral nervous system.